CHICAGO - The government's largest-ever study of Hispanics' health may help answer why they live longer than other Americans, but the first results suggest that for some, the trend might be in jeopardy.

Overall, high rates of high blood pressure, obesity, diabetes and pre-diabetes were found, especially among older adults. But troubling signs were seen among younger Hispanic adults. They were the least likely to have diabetes under control, and the least likely to eat recommended amounts of fruits and vegetables.

Hispanics from Puerto Rico were among the least healthy, while those from South America, who tend to be more recent arrivals, were among the healthiest.

The landmark study is the most comprehensive effort to document the health of U.S. Hispanics. It has followed more than 16,000 Hispanics aged 18 to 74 since 2008.

Mexican Americans are the largest and oldest Hispanic group nationwide, but there has been recent growth among Dominicans and those from Central and South America.

"With the changing face of the Hispanic population, we need more current information about their health," said Dr. Larissa Aviles-Santa, the institute's project director for the study.

Researchers in four cities are documenting the prevalence of chronic disease and risk factors, and trying to determine how adopting U.S. lifestyles affects Hispanics' health. Aviles-Santa said the results could provide a better understanding of what some call the "Hispanic paradox" - longer lives than non-Hispanic white Americans despite some known health risks.

She said the results will help communities find better ways to prevent health conditions afflicting different Hispanic populations.

The other three study sites are Miami, the Bronx in New York, and San Diego.

Hispanics are the nation's largest, fastest-growing ethnic group and make up about 15 percent of the population.

Their life expectancy exceeds whites' by about two years and blacks' by about seven years. Diabetes and obesity are more common in Hispanics than in whites, but Hispanics are less likely to have heart disease, previous data show.

Some experts believe that advantage suggests that the healthiest Hispanics migrate to the United States. But the researchers said that advantage may vanish as unhealthy risk factors accumulate in groups who've been in this country the longest, and in younger adults born in the United States who may be more likely to abandon cultural customs.

"We already know that the longer that people live in the United States, the worse their health becomes," said Neil Schneiderman, the lead investigator for the study's site at the University of Miami.

Study participants were randomly selected. They get free medical exams, fill out health questionnaires and provide other health information periodically during the ongoing study. The first results provide a baseline description of Hispanics' health.

Key findings

Among the findings from the National Heart, Lung and Blood Institute study on the health of U.S. Hispanics:

-- High blood pressure affects almost one-third of Cuban Americans and Puerto Rican Americans, versus one-fifth of those from South America.